Revert "dmaengine: imx-sdma: refine to load context only once"

This reverts commit ad0d92d7, because
in spi-imx case, burst length may be changed dynamically.

Fixes: ad0d92d7 ("dmaengine: imx-sdma: refine to load context only once")
Cc: <stable@vger.kernel.org>
Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Richard Leitner <richard.leitner@skidata.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>